1. Field of the Invention
The invention concerns a method as well as a device for electrostatic charging of both external sides of at least one material web with charges having opposite polarities, prior to the further processing of at least one material web, whereby the at least one material web is guided through the gap of a pair of rollers which are arranged parallel and at a small distance from each other.
2. Description of the Prior Art
Methods and devices for the electrostatic charging of moving webs are known (DE-31 17 419 A1). Several material webs are combined into a strand and only the two external sides are charged by means of a device constructed as corona charging electrodes after the first pair of rollers by means of a high-voltage source of 30 kV.
A disadvantage of this known device is that the charging of the two external sides of the material webs is spatially not well targeted. Thus a large portion of the charged particles stray in the entire region where the charging takes place. The result of this is a poor charging efficiency. In addition, there is the risk that in the case of unfavourable surrounding conditions, the corona charging electrodes become soiled and consequently fail. The cleaning work can be quite considerable.
The invention is a method and a device of the generic type by which a target is charging with a better efficiency.
According to the invention better charging efficiency is achieved with by electrostatic oppositely poled charging of two external sides of at least one material web in the gap between each roller of a roller pair.
On this occasion the charging of the rollers can be carried out via the surface, in fact by means of a so called contact roller (DE-38 23 739 AI), by means of a stroking brush known per se, a sliding contact or a corona charging electrode.
However, in an appropriate development, in the case of the device of the generic type according to the invention the charging is carried out from the inside by the device formed by the pair of rollers, wherein each roller has over a steel jacket a coating with a limited (so called semi-conductive) electrical conductivity and, for the electrical charging of the coating having the limited electrical conductivity, the steel jacket of each roller can be connected to a positive or negative source of high-voltage. A device according to the invention by virtue of the supply of the charging voltage from the inside of the rollers (especially by the retrofitting of existing equipment), makes possible charging of material webs, preferably of plastic material or paper with a high-voltage source of 3 to 7 kV. Thus operation with voltages which are considerable lower than those of the state-of-the-art is possible. In addition, the charging is carried out exactly in that position which is to be charged, namely on the external sides of at least one material web, resulting in an efficiency which is considerably better than that of the state-of-the-art. Finally, if the corona charging electrodes is omitted, a cleaning operation for them is completely dispensable, so that the operational cost of the equipment will be lower due to fewer shutdowns.
In accordance with the invention, the spindle or the shaft of the roller is connected to the steel jacket in an electrically conductive manner and electrically insulated from the machine frame for the rollers. For the electrical charging of the coating with the limited conductivity, the electric high voltage is applied to the spindle or shaft having a terminal for this purpose. This can be constructed either as a fixed terminal on the spindle or as a thrust bearing arranged on the face of the shaft.
In an advantageous embodiment of the invention, it is also feasible that concentrically to the spindle or shaft of the roller and next thereto, a primary coil is mounted stationarily relative to the machine frame and concentrically to the spindle or shaft of the roller on a face thereof a secondary coil is arranged firmly rotating with the roller and one terminal of the secondary coil is connected to the spindle or shaft and the other terminal is connected to the steel jacket via a rectifier circuit.
It is, however, also feasible for the roller to have, for the purpose of insulation from the machine frame, an electric insulator below the highly conductive steel jacket as well as on both of the roller faces. Electrical charging of the coating of the limited conductivity with high voltage from the high voltage source is carried out using a corona charging electrode. Such a roller can also be used as an impression cylinder in the gravure printing process.